US President Barack Obama has sent out a stark warning to Syria, saying any use of chemical weapons in the country's conflict is unacceptable.

It comes amid media reports, citing European and US officials, that Syria's chemical weapons had been moved and could be prepared for use in response to dramatic gains by rebels fighting to topple President Bashar al-Assad.

Rebel forces have made advances in recent weeks, seizing military bases including some close to the capital Damascus.

Obama told Assad if chemicals were used against the opposition forces "there will be consequences".

''The world is watching,'' Obama said in remarks to a gathering of nuclear proliferation experts.

''The use of chemical weapons is and would be totally unacceptable and if you make the tragic mistake of using these weapons there will be consequences and you will be held accountable,'' Obama said.

Officials in Damascus earlier denied Syria would not use such weapons.

"Syria has stressed repeatedly that it will not use these types of weapons, if they were available, under any circumstances against its people," the foreign ministry said.

The opposition believe that Assad, who has upped his response to rebel gains in the 20-month-old revolt, could turn to heavier weapons and some have suggested he might use chemical weapons.

The rebels have begun to advance quickly in recent weeks after months of slow sieges to cut off army routes and supplies.

In the past few weeks, they seized several military bases around the country, and an oil field and hydro-electric dam in the northeast. Rebels are using anti-aircraft weapons to attack the military helicopters and fighter jets that have bombarded their positions with impunity until now.

ASSAD ALLY 'FLEES'

A Syrian foreign ministry spokesman, who was the most public face of Bashar al-Assad's government as it battled a 20-month-old uprising, has reportedly fled the country.

Jihad al-Makdissi, who is in his 40s, previously worked at the Syrian embassy in London and returned to Damascus a year ago to serve as spokesman for the ministry, defending the government's crackdown on the revolt against Assad's rule.

He had little influence in a system largely run by the security apparatus and the military. But Assad's opponents will see the loss of such a high profile figure, if confirmed, as further evidence of a system crumbling from within.

Makdissi belongs to Syria's Christian minority, which has largely stood behind Assad. He worked with the foreign ministry for 10 years and speaks fluent English, a rarity in a state apparatus shaped by the Baath Party's anti-Western ideology.

"He defected. All I can say is that he is out of Syria," the diplomatic source, who did not want to be named, told Reuters.

Lebanon's al-Manar Television, citing government sources, said Makdissi was sacked for making statements that did not reflect the government's position.

He was rarely seen in the media in recent weeks. His mobile telephone was switched off and there was no immediate comment in Syrian state media. The pan-Arab news channel Al Arabiya said Makdissi had left Beirut and was on his way to London, where he was expected to remain.

UN BEGINS PULL OUT

The United Nations has suspended its aid operations in Syria and withdrawing all non-essential international staff due to the worsening security situation.

Up to 25 of about 100 foreign staff could leave this week, it said, adding that more armoured vehicles were needed after attacks in recent weeks on humanitarian aid convoys and the hijacking of goods or vehicles.

Some convoys had been caught in crossfire between Syrian government and rebel forces, including an incident near the airport in which two staff were injured, it said.

"We can confirm that the United Nations in Syria will pull out non-essential international personnel with immediate effect," UN Secretary-General Ban Ki-moon's spokesman Martin Nesirky told reporters.

"The UN will also suspend its missions within the country until further notice," he said in New York.

In all, the world body deploys more than 1,000 national and international staff in Syria, but movement and communications have become more difficult due to intensified fighting near the capital and a 48-hour internet blackout last week, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said.

DAMASCUS BOMBARDMENT

The main focus for the army in the past five days has been Damascus, where security forces are pushing back hard against the rebels and trying to seal the capital off from rebel-dominated suburbs.

The opposition-linked Syrian Observatory for Human Rights said the Syrian army was trying to take over Daraya, on the southern outskirts of Damascus, and was attacking rebels with rockets as it advanced into some parts of the town.

A Syrian security source said that the army had blocked three entrances into Daraya and was optimistic it could take the town. Rebels said they would be able to hold their ground.

"There have been several attempts to storm Daraya and each time the army has suffered major losses. This is not new," said activist Samir al-Shami, of the Syrian Youth Union in Damascus.

Other activists reported heavy bombardment of the towns of Deir al-Asafir and Beit Saham, which are close to the highway leading to Damascus International Airport, the scene of three days of heavy clashes that effectively closed the airport.

Rebels had been planning an advance on the capital, Assad's power base.

The army struck back around the airport last Thursday and since then the suburbs of Damascus have been rocked by fierce clashes and heavy shelling. Activists described continuous shelling that killed more than 56 people around Damascus. More than 200 people died across Syria on Sunday, according to the Observatory.

Neither side has the upper hand in the fighting around Damascus. A previous attempt by rebels last July to hold ground in the city was crushed, but the fighters fell back into the suburbs and nearby countryside.

The Observatory reported air and artillery bombardment in towns across Syria on Monday.

An air strike on the northern border town of Ras al-Ain, which it said killed at least 12 people and wounded more than 30, prompted Turkey to scramble fighter jets along the border.

More than 40,000 people have died in the conflict, with hundreds more killed each week.